This invention relates to the reduction of pollutants produced by boilers, and, more particularly, to the control of the process for reducing NOx pollutants in flue gas flows.
In a fossil-fuel power plant, coal, gas, or oil is burned to boil water to form steam. The steam drives a turbine and thence an electric generator, producing electricity. Besides heat, the combustion produces gaseous pollutants such as sulfur and nitrogen oxides, and a solid particulate termed fly ash. Environmental protection laws mandate that the amounts of the gaseous and solid pollutants be maintained at acceptably low levels. The present invention deals with reducing and maintaining the smog-producing nitrogen oxides, known generally as NOx, within acceptable levels.
It is known that the NOx level in flue gas is lowered by reacting the NOx with ammonia, to produce harmless nitrogen and water as reaction products. The reaction can occur at relatively high temperatures without a catalyst, or at lower temperatures in the presence of a catalyst. The former is known in the art as selective non-catalytic reduction (SNCR), and the latter is known as selective catalytic reduction (SCR). In a process modification, both SNCR and SCR may be performed simultaneously on the same flue gas stream. The SNCR is accomplished in a first zone shortly after the hot flue gas leaves the furnace, and the SCR in a second zone through which the cooler flue gas subsequently passes.
Ammonia must be present for both SNCR and SCR reactions to occur. Since providing a sufficient amount of gaseous or liquid ammonia for both reactions is expensive, the required ammonia molecules may be provided by introducing a nitrogenous compound such as urea into the hot flue gas in the first treatment zone. The urea decomposes at high temperatures, providing ammonia for the SNCR reaction. The urea is usually added in excess of that required for the SNCR reaction, and the SCR helps to remove the resulting excess ammonia from the flue gas stream. Additional nitrogenous compounds or ammonia itself can be added to the flue gas at the second treatment zone, if required to complete the SCR reaction.
Although SNCR and SCR are generally effective in reducing NOx content of the flue gas, the addition of ammonia leads to another potential source of pollution. If the ammonia introduced into the flue gas is not entirely consumed in the reactions with NOx, some ammonia remains in the flue gas and passes to the atmosphere, a consequence termed "ammonia slip". Ammonia slip is often observed in the exhaust plumes of those power plants that use SNCR or SCR to reduce the NOx content of the flue gas.
There is a need for some approach to achieving reduction of NOx in flue gas to acceptably low limits, and also maintaining ammonia slip below the regulatory limits. The present invention fulfills this need, and further provides related advantages.